Electronic circuits

ABSTRACT

A high voltage AC power supply circuit for a capacitive load C L , such as an electroluminescent lamp, includes an inductive component T and an output FET S 1  in series. The output FET S 1  can be pulsed so that the inductive component T generates a voltage to charge the capacitive load C L  via an H-bridge H. A diode D 1  prevents current discharging from the capacitive load C L  while the output FET S 1  Is closed. The circuit also includes a reservoir capacitor C R  and a reservoir FET S 3  in series with the inductive component T. The reservoir FET S 3  can be pulsed so that the inductive component T generates a voltage to charge the reservoir capacitor C R  by transferring energy from the capacitive load C L . A diode D 2  Prevents current discharging from the reservoir capacitor C R  while the reservoir FET S 3  is closed. Energy can be transferred from the capacitive load C L  to the reservoir capacitor C R  and back again to reduce the amount of energy that must be discarded during operation of the circuit.

[0001] The present invention relates to electronic circuits, and inparticular to electronic circuits which can be used in a high voltagepower supply for a capacitive load, such as an electroluminescent lamp.

[0002] Electroluminescent lamps generally comprise a layer of phosphormaterial, such as a doped zinc sulphide powder, between two electrodes.It is usual for at least one electrode to be composed of a transparentmaterial, such as indium tin oxide (ITO), provided on a transparentsubstrate, such as a polyester or polyethylene terephthalate (PET) film.The lamp may be formed by depositing electrode layers and phosphorlayers onto the substrate, for example by screen printing, in which caseopaque electrodes may be formed from conductive, for examplesilver-loaded, inks. Examples of electroluminescent devices aredescribed in WO 00/72638 and WO 99/55121.

[0003] An electroluminescent lamp of the general type described above isilluminated by applying an alternating voltage of an appropriatefrequency between the electrodes of the lamp to excite the phosphor.Commonly, the phosphors used in electroluminescent lamps require avoltage of a few hundred volts. Typically, such electroluminescent lampsmay have a capacitance in the range 100 pF to 1 μF.

[0004] The inventors have been involved in the development ofelectroluminescent displays which comprise electroluminescent lampshaving selectively illuminable regions for displaying information. Suchdisplays have the advantage that they can be large, flexible andrelatively inexpensive. In the context of such electroluminescentdisplays, the inventors have sought to provide a simple power supplyarrangement for an electroluminescent lamp or display.

[0005] A known type of circuit for producing a higher output voltagefrom a low voltage DC supply is a “flyback converter”. Such a circuitcomprises an inductor and an oscillating switch arranged in series. Inparallel with the oscillating switch, a diode and a capacitor arearranged in series. The switch oscillates between an open state and aclosed state. In the closed state, a current flows from the DC supplythrough the inductor and the switch. When the switch is opened, thecurrent path is interrupted, but the magnetic field associated with theinductor forces the current to keep flowing. The inductor thereforeforces the current to flow through the diode to charge the capacitor.The diode prevents the capacitor discharging while the switch is closed.The capacitor can therefore be charged to a voltage which is higher thanthe DC supply voltage, and current at this voltage can be drawn from thecapacitor.

[0006] In order to supply an alternating current to a load from aflyback converter, an H-bridge may be provided in parallel with thecapacitor. In general, an H-bridge comprises two parallel limbs, eachlimb having a first switch in series with a second switch. On each limbbetween the first and second switches there is a node, and the load isconnected between the respective nodes of the limbs. Current can flowthrough the load in one direction via the first switch of one limb andthe second switch of the other limb and in the other direction via theother two switches. The switches of the H-bridge are operated so thatcurrent flows through the load first in one direction and then in theother.

[0007] When an H-bridge is used to supply a capacitive load C_(L) with asupply voltage V, during the first half of the cycle of operation, theload C_(L) is at +V. When the H-bridge switches and reverses thepolarity of the load, there is a potential difference of −2V between thesupply voltage and the load. The load is supplied rapidly with currentfrom the supply until there is no potential difference, and thisrequires 2C_(L)V² of energy. Similarly, when the H-bridge is switched toreturn the load to the original polarity at the end of the cycle, afurther 2C_(L)V² of energy is required to bring the load back to +V.

[0008] It will be seen, therefore, that each cycle of the operation ofthe H-bridge requires 4C_(L)V² of energy. The power consumption,assuming 100% efficiency, is 4C_(L)V²f, where f is the cycling frequencyof the H-bridge. This represents a significant power consumption whenthe frequency and the voltage are large.

[0009] It is usual to provide a large smoothing capacitor (such as thecapacitor of the flyback converter described above) in parallel with theH-bridge in order to provide current for the rapid charging anddischarging of the capacitive load. The smoothing capacitor protects thepower supply from the large currents which result from the switching ofthe polarity of the H-bridge, and ensures that the supply voltage doesnot drop significantly.

[0010] When the polarity of the H-bridge is switched, energy is consumedin recharging the capacitive load. The inventors have sought to reducethis energy consumption.

[0011] The present invention provides an electronic circuit forproviding a high voltage supply to a capacitive load, such as anelectroluminescent lamp, wherein:

[0012] the circuit comprises an inductive element and an outputswitching element arranged in series;

[0013] the output switching element is operable to alternate, in use,between a first state and a second state, whereby in the first state acurrent path is provided through the inductive element and the outputswitching element, which current path is interrupted in the secondstate, such that when the output switching element changes from thefirst state to the second state, the inductive element generates avoltage at an output of the circuit for charging a capacitive load;

[0014] the circuit comprises an output diode arranged to prevent currentflowing back from the output while the output switching element is inthe first state;

[0015] the circuit further comprises a reservoir capacitor, a reservoirswitching element in series with the inductive element, and a reservoirdiode;

[0016] the reservoir switching element is operable to alternate betweena first state and a second state, whereby in the first state a currentpath is provided from the capacitive load through the inductive elementand the reservoir switching element, which current path is interruptedin the second state, such that when the reservoir switching elementchanges from the first state to the second state the inductive elementgenerates a voltage to charge the reservoir capacitor; and

[0017] the reservoir diode is arranged to prevent current flowing backfrom the reservoir capacitor while the reservoir switching element is inthe first state,

[0018] whereby energy can be transferred from the capacitive load to thereservoir capacitor by means of the inductive element and the reservoirswitching element, and energy can be transferred from the reservoircapacitor to the capacitive load by means of the inductive element andthe output switching element.

[0019] Thus, in accordance with the invention, energy stored in thecharged capacitive load can be recovered and stored in the reservoircapacitor, so that the overall energy consumption of the circuit isreduced compared to known flyback converter arrangements.

[0020] The reservoir capacitor may have any suitable capacitance.However, preferably, the reservoir capacitor has a capacitance which isgreater than that of the capacitive load. This has the advantage thatthe energy stored in the capacitive load can be transferred to thereservoir capacitor and stored at a much lower voltage, which reducesthe power loss in charging the reservoir capacitor. The reservoircapacitor may be at least 10 times or preferably at least 100 times thecapacitance of the capacitive load.

[0021] The inductive element may be any suitable component which iscapable of operating in the required manner. Typically, the inductiveelement may have an inductance in the range 50 μH to 50 mH, for example470 μH.

[0022] In a simple embodiment, the inductive element may be an inductoror coil. In a preferred arrangement, however, the inductive element is atransformer. The provision of a transformer has the advantage that thetransfer of energy between the part of the circuit which includes thereservoir capacitor and the part of the circuit which includes thecapacitive load can be achieved by the interaction of the magneticfields of the two sides of the transformer. In this way, direct currentflow from the capacitive load to the reservoir capacitor and vice versais not possible which means that the circuit can be implemented withouta switching arrangement to regulate such current flow.

[0023] The transformer may have substantially identical primary andsecondary windings. However, advantageously, the secondary winding whichis electrically connected to the capacitive load has more turns than theprimary winding. In this way, the transformer acts to step up thevoltage which is transferred from the reservoir capacitor to thecapacitive load and to step down the voltage which is transferred fromthe capacitive load to the reservoir capacitor. The ratio of turns ofthe primary to secondary winding may be in the range 1 to 100 and isgenerally greater than 10.

[0024] The output switching element may be arranged in series with onewinding of the transformer and the reservoir switching element may bearranged in series with the other winding of the transformer.

[0025] The output diode may be any suitable device which allows currentflow in one direction only over the range of operating voltages of thecircuit and the term “diode” is used herein accordingly. The role of theoutput diode is to allow a higher voltage than the DC supply voltage tobe stored on the capacitive load without current flowing back from thecapacitive load towards the inductive element. The reservoir diode maybe any suitable device which allows current flow in one direction onlyover the range of operating voltages of the circuit. The role of thereservoir diode is to allow a higher voltage than the DC supply voltageto be stored on the reservoir capacitor without current flowing backfrom the reservoir capacitor towards the inductive element.

[0026] The output and reservoir switching elements may be any suitableswitching devices and, in general, are transistors. In the preferredarrangement, the switching elements are field effect transistors (FETs).

[0027] In a particularly preferred arrangement, the output and reservoirswitching elements are n-channel FETs.

[0028] In a particularly advantageous arrangement, the output diode maybe arranged in parallel with the reservoir switching element. Inparticular, the output diode and the reservoir switching element may bein the form of a single field effect transistor. In this case, theoutput diode is provided by the parasitic diode which is inherent in theconstruction of a field effect transistor.

[0029] Similarly, the reservoir diode may be arranged in parallel withthe output switching element. In particular, the reservoir diode and theoutput switching element may be in the form of a single field effecttransistor. In this case, the reservoir diode is provided by theparasitic diode which is inherent in the construction of a field effecttransistor.

[0030] Advantageously, the output switching element and/or the reservoirswitching element may be connected directly to earth potential.According to this arrangement, the switching elements are not requiredto be able to switch at high voltage, which simplifies the design of thecircuit.

[0031] The operation of the output and/or reservoir switching elementsmay be controlled by any suitable means. In a preferred arrangement, acontrol voltage is applied to the respective switching element, forexample to the gate of the FET. The control voltage may be a pulse widthmodulated signal. Typically, the frequency of the control voltage is inthe range of 10 to 100 kHz. The circuit may further comprise anoscillator arranged to generate the control voltage.

[0032] The circuit according to the present invention may be used todirectly supply a capacitive load with a varying voltage. However, in apreferred arrangement, the circuit is provided with an H-bridge in orderto supply alternating current to the capacitive load.

[0033] Thus, the circuit may comprise an H-bridge having two parallellimbs, each limb having a first switching element in series with asecond switching element and a node between the first and secondswitching elements, the capacitive load being connected, in use, betweenthe respective nodes of the limbs. The switching elements of theH-bridge may be controlled alternately such that in a first conditionthe first switching elements of one limb and the second switchingelements of the other limb conduct to supply current from the output tothe capacitive load in one direction, and in a second condition theother two switching elements of the limbs conduct to supply current fromthe output to the capacitive load in the opposite direction.

[0034] A smoothing capacitor may be provided in parallel with theH-bridge in order to compensate for the imperfect switching of theswitching elements of the H-bridge. However, the capacitance of theswitching capacitor is desirably kept small, for example less than 50%of the capacitance of the capacitive load, preferably between 10% and20% of the capacitance of the capacitive load.

[0035] The switching elements of the H-bridge may be any suitableswitching devices and, in general, are transistors. In the preferredarrangement, the switching elements are field effect transistors (FETs).In a particularly preferred arrangement, the first switching elementsare p-channel FETs and the second switching elements are n-channel FETs.

[0036] The operation of the switching elements of the H-bridge may becontrolled by any suitable means. In a preferred arrangement, a polarityvoltage is applied to the switching elements, for example to the gatesof the FETs. The polarity voltage may be a pulse width modulated signal.Thus, the circuit may further comprise an oscillator arranged togenerate the polarity voltage. In a particularly convenient arrangement,the signal from the oscillator may also be used to generate the controlvoltage for the reservoir switching element and/or the output switchingelement in order to provide synchronised operation of the converter andthe H-bridge, optionally by means of a divider. Typically, the frequencyof the polarity voltage is in the range 50 Hz to 10 kHz.

[0037] The circuit according to the invention is particularly usefulwhen used in combination with an H-bridge arrangement, because thepolarity of the H-bridge can be switched while energy from thecapacitive load is stored in the reservoir capacitor. In this way, theH-bridge can be switched while there is little or no voltage across thecapacitive load which reduces energy losses and significantly simplifiesthe design of the circuit.

[0038] The circuit may be arranged to operate in accordance with thefollowing steps:

[0039] a) the H-bridge is switched to the first condition;

[0040] b) energy from the reservoir capacitor is transferred to thecapacitive load by means of the inductive element and the outputswitching element;

[0041] c) energy from the capacitive load is transferred to thereservoir capacitor by means of the inductive element and the reservoirswitching element;

[0042] d) the H-bridge is switched to the second condition;

[0043] e) energy from the reservoir capacitor is transferred to thecapacitive load by means of the inductive element and the outputswitching element;

[0044] f) energy from the capacitive load is transferred to thereservoir capacitor by means of the inductive element and the reservoirswitching element.

[0045] The steps a) to f) may be repeated to drive the capacitive loadwith an alternating voltage.

[0046] Current may be supplied to the reservoir and/or the capacitiveload from a DC supply to compensate energy losses in the circuit. Inparticular, the capacitive load may initially be charged from the DCsupply by means of the inductive element and the output switchingelement.

[0047] Typically, the DC supply has a voltage of less than 100 V, forexample in the range 2 to 24 V. The capacitive load may be charged to apeak voltage between 5 to 500 times that of the supply voltage.Typically, the peak voltage is in the range 10 to 100 times that of thesupply voltage.

[0048] The output switching element may be arranged to alternate betweenthe first and the second state at the same frequency as the reservoirswitching element. However, the output switching element may be arrangedto alternate between the first state and the second state at a frequencywhich is different to the frequency at which the reservoir switchingelement alternates between the first and the second state. The outputswitching element may be arranged to alternate between the first and thesecond state at a frequency which is a multiple of the frequency atwhich the H-bridge alternates between the first condition and the secondcondition. In this way, the switching signal to the switching elementsof the converter and the H-bridge can be generated from the sameoscillator, for example using a divider.

[0049] In the preferred arrangement, the capacitive load is anelectroluminescent lamp.

[0050] Some embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

[0051]FIG. 1a and FIG. 1b represent the operation of an H-bridge for usewith the invention;

[0052]FIG. 2a and FIG. 2b illustrate the operation of a flybackconverter;

[0053]FIGS. 3a to 3 c illustrate the operation of an embodiment of theinvention;

[0054]FIG. 4 illustrates the configuration of an alternative form of theembodiment of FIG. 3;

[0055]FIGS. 5a to 5 c illustrate the operation of a further embodimentof the invention; and

[0056]FIG. 6 illustrates the operation of a preferred embodiment of theinvention.

[0057] In the embodiments described, corresponding reference signs havebeen used to indicate corresponding components.

[0058] Referring to FIG. 1a, there is shown an electronic circuitincorporating an H-bridge. The circuit comprises a current source I inseries with a diode D and an H-bridge arrangement H. A smoothingcapacitor C_(S) is provided in parallel with the H-bridge arrangement Hand is connected to earth potential.

[0059] The H-bridge arrangement H in FIG. 1a comprises four switchelements S_(A) to S_(D) which are represented as simple switches forreasons of clarity. In a practical circuit, the switches S_(A) to S_(D)are provided by field effect transistors (FETs). The H-bridge comprisestwo parallel limbs each having two switches S_(A), S_(D) and S_(C),S_(B) arranged in series. A capacitive load C_(L) in the form of anelectroluminescent lamp is connected between the limbs of the H-bridgeat nodes on each limb which are between the switches of the limb. TheH-bridge is connected to earth potential at one end.

[0060] The positions of the switches S_(A) to S_(D) are controlled bytepolarity voltage V_(P), the variation of which over time is representedin FIGS. 1a and 1 b. When V_(P) is low, switches S_(A) and S_(B) areopen and do not conduct and switches S_(C) and S_(D) are closed andconduct. This situation is shown in FIG. 1a. When V_(P) is high,switches S_(A) and S_(B) are closed and conduct while switches S_(C) andS_(D) are open and do not conduct. This situation is illustrated in FIG.1b.

[0061] The operation of the circuit shown in FIGS. 1a and 1 b will nowbe described. A converter such as a flyback converter or forwardconverter, represented as a current source I, supplies current via thediode D to the smoothing capacitor C_(S) and the capacitive load C_(L).The direction in which the capacitive load C_(L) is charged isdetermined by the position of the switches S_(A) to S_(D). Thecapacitors C_(S) and C_(L) continue to be charged until the currentsource I ceases to supply current. The voltage on the capacitors C_(S)and C_(L) consequently rises. Reverse current flow from the capacitorsis prevented by the diode D.

[0062] Thus, when the capacitive load C_(L) is fully charged to the loadvoltage V_(l), the charge thereon is C_(L)V_(L) and the charge on thesmoothing capacitor is C_(S)V_(L). When the polarity voltage V_(P) goeshigh, as shown in FIG. 1b, the polarity of the charged capacitive loadC_(L) with respect to the smoothing capacitor C_(S) and the currentsource is reversed. Thus, point Y in FIG. 1b is at a potential −V_(L)relative to earth potential, while the potential at point X is +V_(L)relative to earth potential. This potential difference causes current toflow until points X and Y are at the same potential.

[0063]FIGS. 2a and 2 b show an arrangement of a flyback converter forcharging a capacitive load to a high voltage. The flyback convertershown in FIG. 2 can be used with the H-bridge arrangement H shown inFIG. 1, although for the sake of simplicity the capacitive load C_(L) isshown in FIG. 2 without the H-bridge.

[0064] As shown in FIG. 2a, the flyback converter comprises a DC supplyin series with an inductor L and a switch S. The switch S is connectedbetween the inductor and earth potential. In a practical arrangement,the switch S is provided by a field effect transistor. However, for thesake of clarity, in FIGS. 2a and 2 b the switch S is shown as a simpleswitch.

[0065] In parallel with the switch S is provided a diode D in serieswith the capacitive load C_(L). The capacitive load C_(L) is arrangedbetween the diode and earth potential.

[0066] The switch S is controlled by a switch voltage V_(S) which variesover time as indicated in FIG. 2a. When the switch voltage V_(S) ishigh, the switch S is closed and conducts. This situation is shown inFIG. 2a. When the switch voltage V_(S) is low, the switch S is open anddoes not conduct. This situation is shown in FIG. 2b.

[0067] The circuit shown in FIGS. 2a and 2 b operates as follows. Whilethe switch voltage V_(S) is high, as shown in FIG. 2a, current I flowsfrom the DC supply through the inductor L and the closed switch S toearth. Assuming the voltage on the capacitive load C_(L) is higher thanthe DC supply voltage, no current flows through the diode D.

[0068] When the switch voltage V_(S) goes low, as shown in FIG. 2b; thecurrent path through the inductor L and switch S is interrupted by theopen switch S. However, the energy stored in the magnetic fieldassociated with the inductor L forces the current I to continue flowingand the inductor L generates a sufficiently high voltage that thecurrent I flows through the diode D to charge the capacitive load C_(L).In this way, with each transition of the switch voltage V_(S) from highto low, the voltage V_(L) on the capacitive load C_(L) is increased, asindicated in FIG. 2b. The diode D prevents current flow back from thecapacitive load C_(L) to earth or to the DC supply when the switch S isclosed.

[0069] It will be seen therefore that the capacitive load C_(L) can becharged to any desired voltage by applying an alternating switch voltageV_(S) to the switch S.

[0070]FIGS. 3a to 3 c show an improved flyback converter according tothe invention. Again, this configuration can be used with the H-bridgeshown in FIG. 1, but for simplicity the circuit is shown directlyconnected to the capacitive load C_(L). This arrangement is similar tothat of FIG. 2 in that this arrangement comprises a DC supply in serieswith an inductor L. The inductor L is also in series with a switch S₁,connected between the inductor L and earth potential, which correspondsto the switch S in FIG. 2. In parallel with the switch S₁ are an outputdiode D₁, which corresponds to the diode D of FIG. 2, and the capacitiveload C_(L). The capacitive load C_(L) is connected on one side to earthpotential.

[0071] In addition to the components corresponding to those of thecircuit shown in FIG. 2, the arrangement of FIG. 3 also includes anoutput diode bypass switch S₂, which, when closed, bypasses the outputdiode D₁ to allow current flow from the capacitive load C_(L) to theinductor L.

[0072] Between the DC supply and the inductor L is provided anarrangement of components which substantially mirrors the arrangement ofthe capacitive load C_(L), output diode D₁, output diode bypass switchS₂ and output switch S₁. Thus, a reservoir switch S₃ is provided betweenthe inductor L and earth potential. In parallel with the reservoirswitch S₃ is a reservoir capacitor C_(R) which is also connected on oneside to earth potential. Between the DC supply and the inductor L isarranged a reservoir diode D₂ to prevent current flow from the reservoircapacitor C_(R) through the inductor L. A reservoir diode bypass switchS₄ is provided in parallel with the reservoir diode D₂ in order toselectively permit discharge of the reservoir capacitor C_(R) throughthe inductor L. A supply switch S₅ is provided in series with the DCsupply to selectively enable or disable supply of current to thecircuit.

[0073] The circuit shown in FIGS. 3a to 3 c is capable of charging thecapacitive load C_(L) to a voltage which is higher than that of the DCsupply and then discharging the capacitive load C_(L) so that energytherefrom is stored in the reservoir capacitor C_(R). The capacitiveload C_(L) can then be recharged from the reservoir capacitor C_(R). Inthis way, the capacitive load C_(L) can be charged and dischargedwithout significant wastage of energy.

[0074] The circuit shown in FIGS. 3a to 3 c operates as follows. Asshown in FIG. 3a, the supply switch S₅ and the reservoir diode bypassswitch S₄ are closed to provide a current path from the DC supplythrough the inductor L. The reservoir switch S₃ and the output diodebypass switch S₂ are open. It will be seen therefore that the circuit inthis condition is substantially electrically equivalent to the circuitshown in FIGS. 2a and 2 b. Thus, the output switch S₁ is pulsed betweenan open and closed position in order to charge the capacitive load C_(L)to a desired voltage in a corresponding manner to that described inrelation to FIGS. 2a and 2 b.

[0075] To discharge the capacitive load C_(L), the supply switch S₅ andthe reservoir diode bypass switch S₄ are held open. The output switch S₁is held open and the output diode bypass switch S₂ is closed so thatthere is a current path from the capacitive load C_(L) through theinductor L. The reservoir switch S₃ is pulsed in order to charge thereservoir capacitor C_(R) in the manner described in relation to FIG. 2while drawing current from the capacitive load C_(L).

[0076] The capacitive load C_(L) is recharged from the reservoircapacitor C_(R) as shown in FIG. 3c. In this case, the arrangement ofthe switches S₁ to S₄ is identical to that in FIG. 3a when thecapacitive load is charged from the DC supply. However, in this case,the supply switch S₅ is held open so that current is not drawn from theDC supply.

[0077] Thus, it will be appreciated that the circuit shown in FIG. 3 iscapable of charging and discharging a capacitive load without discardingenergy from the load.

[0078]FIG. 4 shows an alternative configuration of the embodiment ofFIG. 3 which does not require connection to earth potential. Accordingto this configuration, a connection is made between the capacitive loadC_(L) and the reservoir switch S₃ and a connection is made between thereservoir capacitors C_(R) and the output switch S₁. The operation ofthe circuit is similar to that of the circuit shown in FIGS. 3a to 3 cwith the exception that in this arrangement the output switch S₁ and thereservoir switch S₃ are arranged to operate in antiphase, so that whenone is open, the other is closed and vice versa.

[0079]FIGS. 5a to 5 c show a further embodiment of the invention inwhich the inductor L is replaced by a transformer T. This arrangementhas the advantage that there is no direct current path between thereservoir capacitor C_(R) and the capacitive load C_(L), which reducesthe number of switches that are required in the circuit.

[0080] The circuit comprises two halves linked inductively by thetransformer T. One half of the circuit comprises the reservoir capacitorC_(R), the primary winding of the transformer T and the output switch S₁in series. The reservoir diode D₂ is provided in parallel with theoutput switch S₁. The DC supply, in series with the supply switch S₅, isprovided in parallel with the reservoir capacitor C_(R).

[0081] The other half of the circuit comprises the capacitive load C_(L)in series with the secondary winding of the transformer T and thereservoir switch S₃. The output diode D₁ is provided in parallel withthe output switch S₃. The primary and secondary windings are arrangedsuch that the current induced in the secondary winding is in theopposite sense to that in the primary winding.

[0082] The operation of the circuit shown in FIGS. 5a to 5 c is asfollows. As shown in FIG. 5a, to charge the capacitive load C_(L) fromthe DC supply, the supply switch S₅ is closed and the reservoir switchS₃ is open. The output switch S₁ is pulsed so that energy is transferredfrom the reservoir side of the transformer to the capacitive load C_(L)by inductive coupling of the windings of the transformer T. In this way,the capacitive load C_(L) is charged to a high voltage.

[0083] To discharge the capacitive load C_(L), the supply switch S₅ isheld open, the output switch S₁ is held open and the reservoir switch S₃is pulsed so that energy is transferred from the capacitive load C_(L)to the reservoir capacitor C_(R) via inductive coupling in thetransformer T. This situation is shown in FIG. 5b.

[0084] To transfer energy from the reservoir capacitor C_(R) to thecapacitive load C_(L) the supply switch S₅ is held open, the reservoirswitch S₃ is held open and the output switch S₁ is pulsed so that energyis transferred by inductive coupling in the transformer T from thereservoir capacitor C_(R) to the capacitive load C_(L).

[0085] It will be seen that the simple arrangement in FIG. 5 allows thecapacitive load C_(L) to be charged to a high voltage and energy fromthe capacitive load C_(L) to be transferred back to the reservoircapacitor C_(R) so that energy wastage is minimised.

[0086]FIG. 6 shows a circuit in accordance with a preferred embodimentof the invention. The circuit combines the features of the arrangementof FIG. 5 and the H-bridge of FIG. 1.

[0087] The circuit shown in FIG. 6 comprises a reservoir capacitor C_(R)having a capacitance of approximately 1 μF in series with the primarywinding of a transformer T and an n-channel FET. The n-channel FETprovides the output switch S₁, and also the reservoir diode D₂ by meansof the parasitic diode inherent in the FET construction. The gate of then-channel FET S₁ is supplied with a forward voltage signal V_(F).

[0088] The DC supply is arranged in parallel with the reservoircapacitor C_(R) for supplying a current I_(S).

[0089] The circuit shown in FIG. 6 further comprises another n-channelFET in series with the secondary winding of the transformer T and anH-bridge H. The n-channel FET provides the reservoir switch S₃ and theoutput diode D₁ by means of the parasitic diode of the FET. The gate ofthe FET S₃ is supplied with a reverse voltage V_(R).

[0090] A smoothing capacitor C_(S) is provided in parallel with theH-bridge H and has a capacitance of around 1 nF.

[0091] The H-bridge H comprises two parallel limbs. The first limbcomprises a p-channel FET S_(A) in series with an n-channel FET S_(D).Between the two FETs S_(A) and S_(D) there is a connection for thecapacitive load C_(L), which is an electroluminescent lamp with acapacitance of around 10 nF. The gates of the FETs S_(A) and S_(D) aresupplied with a polarity voltage V_(P). The other limb of the H-bridgecomprises a p-channel FET S_(C) in series with an n-channel FET S_(B).The capacitive load C_(L) is connected to a point between the two FETsS_(C) and S_(B). The gates of the FETs S_(C) and S_(B) are supplied withthe inverse of the polarity voltage V_(P) by means of an inverter INV.

[0092] As indicated by the voltage graphs in FIG. 6, one cycle of thecircuit comprises four distinct phases a, b, c and d. In phase a, thepolarity voltage V_(P) is low, such that FETs S_(A) and S_(B) conductwhile FETs S_(C) and S_(D) do not conduct. The reverse voltage V_(R) islow so that the reservoir FET S₃ does not conduct. The forward voltageV_(F) pulses so that the output FET S₁ alternately conducts and does notconduct. Consequently, the changing current through the primary windingof the transformer T induces a current in the secondary winding tocharge the smoothing capacitor C_(S) and the capacitive load C_(L), viathe FET S_(A). The voltage V_(L) across the capacitive load C_(L) in thedirection of the arrow in FIG. 6 rises due to is the increased charge onthe capacitive load C_(L), as does the voltage V_(HV) at point X.

[0093] In phase b, the forward voltage V_(F) is held low such that theoutput FET S₁ does not conduct. The polarity voltage V_(P) remains lowso that the FETs S_(A) and S_(B) continue to conduct, while the FETsS_(C) and S_(D) do not. The reverse voltage V_(R) pulses so that whenthe reverse voltage V_(R) is high, current flows from the capacitiveload C_(L) via the FET S_(A) through the secondary winding of thetransformer T and through the reservoir FET S₃ to earth. When theforward voltage V_(F) goes low the reservoir FET S₃ ceases to conductwhich causes the energy in the secondary winding of transformer T toforce a current flow in the primary winding to charge the reservoircapacitor C_(R). Consequently, the voltage V_(L) across the capacitiveload C_(L) drops, as does the voltage V_(HV) at point X.

[0094] In phase c, the polarity voltage V_(P) goes high, such that theFETs S_(A) and S_(B) cease to conduct and the FETs S_(C) and S_(D) beginto conduct. The polarity of the capacitive load C_(L) relative to thepoint X is therefore reversed. However, it is to be noted that when thischange of polarity occurs, the charge on the capacitive load C_(L) issmall. In this way, it is unnecessary to draw significant current whenthe polarity of the H-bridge is switched.

[0095] During phase c, the reverse voltage V_(R) is low so that thereservoir FET S₃ does not conduct. The forward voltage V_(F) is pulsedso that current is drawn intermittently from the reservoir capacitorC_(R) through the primary winding of the transformer T to induce acurrent in the secondary winding to charge the capacitive load C_(L).However, because the FETs S_(C) and S_(D) are conducting rather than theFETs S_(A) and S₅, the capacitive load C_(L) is charged with current inthe opposite direction to that in phase a, so that a negative voltagerelative to the voltage V_(HV) at point X is provided on the capacitiveload C_(L).

[0096] In phase d, the capacitive load C_(L) is discharged and theenergy is stored in the reservoir capacitor C_(R) in the same manner asin phase b.

[0097] Between phase d and the repeat of phase a, the polarity voltageV_(P) goes low. Again, this occurs while the voltage on the capacitiveload C_(L) is small, so that it is unnecessary to draw significantcurrent.

[0098] Thus, it will be seen that according to this arrangement there isprovided a simple, energy efficient power supply for anelectroluminescent lamp.

[0099] In summary, a high voltage AC power supply circuit for acapacitive load, such as an electroluminescent lamp, includes aninductive component and an output FET in series. The output FET can bepulsed so that the inductive component generates a voltage to charge thecapacitive load via an H-bridge. A diode prevents current dischargingfrom the capacitive load while the output FET is closed. The circuitalso includes a reservoir capacitor and a reservoir FET in series withthe inductive component. The reservoir FET can be pulsed so that theinductive component generates a voltage to charge the reservoircapacitor by transferring energy from the capacitive load. A diodeprevents current discharging from the reservoir capacitor while thereservoir FET is closed. Energy can be transferred from the capacitiveload to the reservoir capacitor and back again to reduce the amount ofenergy that must be discarded during operation of the circuit.

1. An electronic circuit for providing a high voltage supply to acapacitive load, such as an electroluminescent lamp, wherein: thecircuit comprises an inductive element and an output switching elementarranged in series; the output switching element is operable toalternate, in use, between a first state and a second state, whereby inthe first state a current path is provided through the inductive elementand the output switching element, which current path is interrupted inthe second state, such that when the output switching element changesfrom the first state to the second state, the inductive elementgenerates a voltage at an output of the circuit for charging acapacitive load; the circuit comprises an output diode arranged toprevent current flowing back from the output while the output switchingelement is in the first state; the circuit further comprises a reservoircapacitor, a reservoir switching element in series with the inductiveelement, and a reservoir diode; the reservoir switching element isoperable to alternate between a first state and a second state, wherebyin the first state a current path is provided from the capacitive loadthrough the inductive element and the reservoir switching element, whichcurrent path is interrupted in the second state, such that when thereservoir switching element changes from the first state to the secondstate the inductive element generates a voltage to charge the reservoircapacitor; and the reservoir diode is arranged to prevent currentflowing back from the reservoir capacitor while the reservoir switchingelement is in the first state, whereby energy can be transferred fromthe capacitive load to the reservoir capacitor by means of the inductiveelement and the reservoir switching element, and energy can betransferred from the reservoir capacitor to the capacitive load by meansof the inductive element and the output switching element.
 2. Anelectronic circuit as claimed in claim 1, wherein the reservoircapacitor has a capacitance which is greater than that of the capacitiveload.
 3. An electronic circuit as claimed in claim 1 or 2, wherein theinductive element is a transformer.
 4. An electronic circuit as claimedin any preceding claim, wherein the output diode is arranged in parallelwith the reservoir switching element.
 5. An electronic circuit asclaimed in claim 4, wherein the output diode and the reservoir switchingelement are in the form of a single field effect transistor.
 6. Anelectronic circuit as claimed in any preceding claim, wherein thereservoir diode is arranged in parallel with the output switchingelement.
 7. An electronic circuit as claimed in claim 6, wherein thereservoir diode and the output switching element are in the form of asingle field effect transistor.
 8. An electronic circuit as claimed inany preceding claim, wherein the output switching element and/or thereservoir switching element is connected directly to earth potential. 9.An electronic circuit as claimed in any preceding claim, wherein thecircuit further comprises an H-bridge connected to the output and havingtwo parallel limbs, each limb having a first switching element in serieswith a second switching element and a node between the first and secondswitching elements, the capacitive load being connected, in use, betweenthe respective nodes of the limbs, wherein the switching elements of theH-bridge are controlled alternately such that in a first condition thefirst switching elements of one limb and the second switching elementsof the other limb conduct to supply current from the output to thecapacitive load in one direction, and in a second condition the othertwo switching elements of the limbs conduct to supply current from theoutput to the capacitive load in the opposite direction.
 10. Anelectronic circuit as claimed in claim 9, wherein the H-bridge isarranged to switch between the first condition and the second conditionwhile energy from the capacitive load is stored in the reservoircapacitor.
 11. An electronic circuit as claimed in claim 9 or 10,wherein the circuit is arranged to operate in accordance with thefollowing steps: a) the H-bridge is switched to the first condition; b)energy from the reservoir capacitor is transferred to the capacitiveload by means of the inductive element and the output switching element;c) energy from the capacitive load is transferred to the reservoircapacitor by means of the inductive element and the reservoir switchingelement; d) the H-bridge is switched to the second condition; e) energyfrom the reservoir capacitor is transferred to the capacitive load bymeans of the inductive element and the output switching element; f)energy from the capacitive load is transferred to the reservoircapacitor by means of the inductive element and the reservoir switchingelement; and g) the steps a) to f) are repeated.
 12. An electroniccircuit as claimed in any preceding claim, wherein current is suppliedto the reservoir and/or the capacitive load from a DC supply tocompensate energy losses in the circuit.